1. Field of the Invention
The present invention relates to a rubber modified polystyrene composition having excellent environmental stress-crack resistance (ESCR) properties, and more specifically, it relates to an oil-resistant rubber modified polystyrene composition.
2. Description of the Prior Art
In general, rubber modified polystyrenes have good moldability and excellent physical properties, and have been used in various applications. However, conventional rubber modified polystyrenes have poor oil resistance and flon resistance, and for this reason, they tend to form environmental stress cracks and exhibit various other problems after only a short period of use. The resistance of various rubber modified polystyrenes to environmental stress-cracks caused by exposure to long chain oils or a fats has been studied, for example in Japanese PCT Patent Application Laid-Open No. 504450/1996 and Japanese Patent Application Laid-Open No. 12845/1996, which describe oil-resistant rubber modified polystyrenes which may be used as margarine containers, trays for refrigerators and the like.
Recently, resins used in food packaging have been required, as much as possible, to have reduced level of residual unreacted monomers and volatile components, such as polymerization solvents. By volatile components, we mean components of the rubber modified polystyrene composition, usually low molecular weight compounds, capable of vaporizing at a temperature of 230xc2x0 C. or less, and/or a pressure of 760 mm Hg or less. Volatile components may therefore include unreacted monomers, dimers and trimers thereof, solvents, water, etc. Because it is now apparent that dimers and trimers of the monomers from which the composition is polymerized may be endocrine disruptors, residual levels of these dimers and trimers must be reduced as much as possible in resins used in food containers, for example, edible oil containers, cups for juices, and the like.
Conventionally, resins having lower amounts of residual volatile components are prepared by heating the polymerization solution up to higher temperatures, or by reducing the pressure during the devolatilization step, so that the unreacted monomer and solvent are devolatilized from the polymerization solution. However, heating the polymerization solution to a higher temperature, decreases the degree of swelling and impairs the oil resistance of the resins thus formed, and the amount of residual dimer and trimer in the resin increases as the treatment temperature increases. In addition, reducing the pressure during devolatilization of the resin, and monomer and solvent recovery have the disadvantage of requiring additional energy.
Accordingly, the present invention provides oil-resistant rubber modified polystyrene composition having reduced levels of residual volatile substances, as well as reduced levels of dimer and trimer. The present inventors have found that the oil-resistance properties of the rubber modified polystyrene are affected by the average volume particle diameter of the rubber phase, and the gel content and degree of swelling of the composition. Moreover, the devolatilization method also affects the oil-resistance properties, the degree of swelling and the level of residual volatile substances, dimer, and trimer.